Wireless communication networks typically comprise many access points (APs) and associated wireless devices. This is particularly so for high density (HD) networks, such as HD wireless local area networks (WLANs). Each AP and its associated wireless devices in the network constitute a basic service set (BSS) and a typical HD network comprises many overlapping BSSs. The increasing popularity of wireless devices, such as mobile telephones, notebook computers, personal digital assistants (PDAs) and the like, place an increasing throughput demand on such networks and therefore motivation exists to increase network capacity.
However, the combined network capacity is limited due to co-channel interference (CCI) and adjacent channel saturation. To improve network capacity in networks based on carrier sensing (CS), Clear Channel Assessment (CCA) is used by wireless communication devices to mitigate interference. CCA samples the energy in the channel to determine whether the air medium is busy by comparing the incoming energy of a signal against a threshold energy level, referred to as the CCA threshold, and consequently controls how transmission and reception of signals should proceed.
In an interference-limited wireless communication network, when CCA is extremely sensitive, the collision rate between signal packets will remain very low. However, the network capacity will also remain low because of too many deferrals due to far-away cell transmission, i.e. exposed terminals. An exposed terminal is unnecessarily prevented from transmitting even though it would not have disrupted the intended transmission. As the sensitivity of CCA is reduced, the collision rate increases. Consequently, the network capacity also increases because an increasing number of transmissions are allowed. As CCA sensitivity is reduced even further, the network capacity will drop as the collision rate becomes extremely high. Hence, one known method of improving network capacity in wireless communication networks involves tuning or adapting the CCA threshold to minimise collisions or the packet error rate (PER).
However, there are some interference scenarios that can not be prevented by self CCA adaptation alone. Adapting the CCA threshold of a transmitter is effective where interference arrives prior to the signal and can be detected by the CCA of the transmitter. However, signal packets will be lost where interference arrives at the same time as the signal, the well-known collision scenario, or where interference arrives after the signal. The probability of collision where interference arrives at the same time as the signal can however be reduced by binary exponential backoff (BEB) mechanisms as defined in the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard. Nonetheless, the problem of reduced network capacity due to interference arriving after the signal, i.e. hidden terminals, remains.
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